In the making of snow for winter sport activities such as down hill skiing the most prevalent means is to use compressed air and water which are supplied to so-called snow guns or nozzles for the atomizing, projection and distribution of the resulting product. In most of the existing nozzles the compressed air and water are mixed internally within the body of the nozzle and are discharged from the nozzle outlet as a mixture. The compressed air provides the energy source for water atomization and also supplies a significant proportion of the momentum necessary to project the droplets and distribute them as frozen particles on the ski slope. The compressed air may serve a secondary purpose in snow making. Depending on the expansion process the compressed air may be cooled and therefore contribute to the snow making by removal of heat from the water. The two phase jet issuing from the nozzle will induce secondary cold ambient air to mix with the primary stream. It is the secondary air and the surrounding atmosphere that provide the largest proportion of the required cooling to convert the water droplets to ice particles.
The ratio of compressed air to water used in snow making can change by more than an order of magnitude even for the same nozzle since the snow making process is highly dependent on the ambient temperature and the humidity of the air. In North America it is customary to quote the ratio in terms of scfm (standard cubic feet per minute) of air per USgpm (US gallons per minute) of water. For calculation purposes a mass ratio of compressed air to water is more meaningful and is used in this application. The practical limits of the mass ratio are 0.01 to 0.5 which would include very efficient units operating at temperatures of -20.degree. C. and colder and relatively inefficient units operating at temperatures approaching 0.degree. C. For comparison a mass ratio of 0.10 is equivalent to a ratio of 11:1 scfm/USgpm. The pneumatic method of snow making is an energy intensive process. The typical compressed air plant supplying air at 100 psig will pump approximately 4.5 scfm per horsepower input, thus for the mass ratio case of 0.10 an energy input of 2.5 hp per gallon water pumped per minute would be required for the compressed air. At the high limit ratio of 0.5 this would mean an input of 12.5 hp per gpm for compressed air.
In order to reduce the energy input for snow making while retaining the compressed air method there is a need to develop more energy-efficient snow making nozzles. The physical processes required of the snow making nozzle are atomization of the water stream and projection of the air-Water mixture with a minimum loss of momentum.
For a specific nozzle, the degree of atomization attained is a function of the supply pressure of the fluids and the mass ratio of the air to water. The mean size of droplets required for snow making depends on several factors including the ambient dry bulb and wet bulb temperatures, the wind velocity and the time of flight of the droplet, all of which affect the heat transfer processes involved. As the mean droplet size is reduced, by increasing the air/water mass ratio, the available surface area increases for a given quantity of water. An increased surface area results in a higher heat transfer within a given time period. At ambient freezing temperatures just below O.degree. C. the droplet size must be minimized so that a high surface area is provided to compensate for the lower heat transfer rate resulting from the small temperature differential available. Smaller droplets also have a lower terminal velocity and thus from a given height, the apogee of their flight, the smaller droplets take longer to contact the ground. The longer time of flight allows for a greater heat transfer.
In the late 1930's a research work was carried out in Japan on the atomizing of fluids by compressed air from which it was established that the droplet sizes produced by internal mix nozzles was a function of the difference in velocity, the slip velocity, between the liquid and the air. The formula developed by Nukiyama and Tarasawa, Experiment On The Atomization Of Liquid By Means Of An Air Stream, Trans. Soc. Mech. Engrs. Japan, Vol. 4, 1938, pp. 86-93, has remained in use although it has been shown that this empirical formula is not dimensionally consistent. Subsequent work by others has extended the application of the formula to larger nozzles with higher flow rates and experimentally to an external atomizing means with a supersonic air nozzle, Atomization Of Liquid By Supersonic Air Jets, Industrial and Engineering Chemistry, Vol. 47, No. 1, 1955, pp. 23-28. It has been demonstrated that higher differential velocities result in smaller droplet sizes. Droplet size also is a linear function of the nozzle orifice diameter when other factors are constant, therefore to increase capacity it is preferable to increase the number of nozzles in preference to increasing the size of a nozzle, Airblast Atomization: The Effect Of Linear Scale On Mean Drop Size, ASME, 1980, Gas Turbine Conf., Paper 80GT74.
With internal mix nozzles there are several methods by which the air and water can be mixed. A large low velocity mixing chamber can be provided into which the air and water must be admitted at approximately the same pressure. Numerous methods have been developed with the aim of producing a homogeneous mixture. From the mixing chamber the air-water mixture is discharged to the atmosphere generally through a converging nozzle. When air is discharged to the atmosphere through a convergent nozzle the maximum velocity that can be attained by the air is equal to the speed of sound and this occurs at the outlet orifice, Compressed Gas Handbook, NASA, SP 3045, 1969. The speed of sound in a homogeneous air-water mixture is much lower than that in air alone thus the maximum velocity that can be attained by the mixture is lower. If the mixture is not homogeneous as is often the case then the two fluids may exit at different velocities. Even with premixing before a convergent or a cylindrical nozzle some separation may take place and usually the flow is coaxial with an inner core that is predominantly gaseous while the outer annular flow is primarily the liquid component. This is one of the known modes of two phase flow, One dimensional Two Phase Flow, McGraw-Hill Book Co., New York, 1969.
For a homogeneous mixture the friction loss is much greater than for either component. For non-homogeneous mixtures that which is predominantly water in contact with the boundary wall has a higher friction than one which is predominantly gaseous at the wall. This would suggest that from an energy efficiency aspect a non-homogeneous mixture with air in contact with the wall would be the preferred mode. One of the methods used in the atomizing of water is the sheet forming process as an initial phase. Water is formed into a sheet on a surface and is then accelerated there by reducing the film thickness until ultimately ligaments and then droplets form. In air atomizing nozzles using the sheet forming technique a high air velocity is desirable in order to provide for the acceleration of the water film. Another method of atomizing water is based on jet instability, Experiments On Liquid Jet Instability, Journal Of Fluid Mechanics, Vol. 40, Part 3, 1970, pp. 495-511, and differential velocity between the water and the surrounding air. This is a method used by some face mixing and external mixing nozzles. These nozzles generally use convergent coaxial air nozzles and thus are limited to the velocity of sound for the discharge air.